CHICAGO — In the soft shimmer of the water near DuSable Harbor, several Canada geese and a double-crested cormorant foraged among aquatic plants, the former nibbling on the feather-like leaves and the latter diving time and again for fish. By the docks and boats, an American black duck preened itself.

Near the geese, a dead bird floated next to trash and vegetation. It likely fell victim to a lurking danger that has killed at least a dozen waterfowl along the lakefront downtown in recent months, according to county and state officials.

The Illinois Department of Natural Resources confirmed that it had received reports of dead mallard ducks in DuSable Harbor and picked up carcasses Aug. 21 for diagnostic testing. The University of Illinois College of Veterinary Medicine Diagnostic Laboratory tested one of the birds and shared with Cook County and the state agency on Sept. 11 that the cause of death was not apparent, and that the test results were inconclusive.

Tissues have been frozen for additional testing. According to an IDNR spokesperson on Monday, the results of histopathology — the study of diseased cells and tissues with a microscope — is pending.

Advocates and other experts, however, suspect that the birds succumbed to avian botulism. It is not an uncommon disease among waterfowl, especially after periods of drought, said Chris Anchor, a senior wildlife biologist with the Forest Preserves of Cook County, which collaborated with the state to conduct pathology exams and inspect the lakefront for more dead birds, from Rainbow Beach on Chicago’s South Side to the Indiana stateline. Most species affected this summer have been mallard ducks, as well as herring and ring-billed gulls.

“Almost every summer, we have cases,” including in city parks and ponds farther inland, said Annette Prince, director of the Chicago Bird Collision Monitors. “There’s no stopping it. It occurs in nature. The one way to slow it down or to improve (it) is to, certainly, clean up everything.”

The volunteer rescue organization has received calls regarding between half a dozen and a dozen suspected cases, including at Ohio Street Beach and near Museum Campus — “obviously, anything along the lakefront is rather connected,” Prince said.

Botulism is caused by a toxin that the bacterium Clostridium botulinum releases in warm water and environments with no oxygen. Since early August, surface water temperatures in Lake Michigan have been in the upper 60s to lower 70s, according to the National Oceanic and Atmospheric Administration.

When drought leads to low water levels, it strands decaying vegetation on beaches. When birds consume that, or small invertebrates and fish infected with the neurotoxin, they become infected as well. The disease is not contagious, but it can still spread quickly through ingestion.

“When they die, here’s where the snowball effect is,” Anchor said.

Flies lay eggs in dead bird carcasses, filling them with maggots that become infected with the toxin. Then, other birds feed on the infected maggots, ingesting concentrated doses of the toxin.

“Then it becomes this cycle of things dying, then getting ingested by the next thing, and then it kind of keeps going until it sort of runs its course,” Prince said.

So-called dabbler birds like geese usually become affected first, since they feed on the surface of bodies of water or close to it, scavenging off the layer of scum.

“It goes from a handful of birds to, suddenly, you’ve got 50 or 100 or 1,000 birds dead, and we’ve seen that,” Anchor said. “And that’s why we’re very anxious to get our hands on as many carcasses as possible, not just for testing, but to get them out of the environment.”

In early August, nearly 14,000 birds died during a botulism outbreak at the Tule Lake National Wildlife Refuge on the Oregon-California border, local media reported.

Symptoms of avian botulism include paralysis, as the toxin causes progressive muscle weakness with drooping wings, weak legs and a limp neck, and often culminates in death by drowning or respiratory failure.

Two types of botulism occur in birds, according to the U.S. Geological Survey. Type C is common during warmer months across the country, while type E is most common in the Great Lakes region during the spring and fall.

Fall bird migration is reaching its peak in North America, and Chicago is in the middle of the country’s largest avian migratory route, the Mississippi Flyway.

Type E botulism emerged as a problem in the region in the 1960s as nutrient runoff concentrated in waterways, producing low-oxygen pockets and optimal conditions for the botulism toxin to form. According to the geological survey, outbreaks have become more widespread in recent years.

Research led by the federal agency and the University of Wisconsin-Madison suggests that rising lake temperatures and more algal blooms could increase avian mortality from botulism in the Great Lakes.

These happen to be among the fastest-warming lakes in the world due to climate change. One of the many consequences of warm surface temperatures, which contribute to low levels by enhancing evaporation, is low water levels. In 2024, Lake Michigan water levels dropped below their long-term averages for the first time in a decade.

When it comes to drought, while total annual rainfall has increased in Illinois — nearly 6 inches between 1895 and 2019, an increase of approximately 15% — precipitation is falling in more extreme, heavier bursts separated by longer, more intense periods of dry weather, according to the Illinois State Climatologist Office.

Some scientists also believe the spread of invasive species might be contributing to widespread avian botulism outbreaks in the region.

Invasive round goby fish — first spotted in the Great Lakes in 1990 — can help transmit the type E toxin that causes botulism. Gobies feed on decaying organic matter at the bottom of the lakes, and birds then consume the infected fish, according to the Wildlife Health Lab at Cornell University’s College of Veterinary Medicine.

And invasive zebra and quagga mussels filter water, allowing more light in. That, in turn, exacerbates algae growth, which eventually depletes oxygen when the vegetation dies and decomposes.

“This isn’t okay, lots of ducks are dying a slow and painful death,” wrote Victoria Helen, a Loop resident, on a Facebook post after seeing a female mallard duck dying in the northeast corner of the harbor while walking her dog last week.

After Helen and other residents said they got the runaround trying to report the deaths and ensure cleanup at the harbor, state officials stepped in to investigate and brought in Cook County’s wildlife experts.

“We work hip-to-hip with DNR and other counties and the feds, because there’s so few of us left that it would be impossible for any one agency to respond by itself,” Anchor said.

He encouraged residents to reach out to the agency or their county’s forest preserves if they encounter dead or dying waterfowl.

“Everything dies, so it’s quite common to find one (dead bird). That’s normal, that’s life,” he said. “But if you see five or more, yeah, then something’s probably cooking. We need to check it out.”

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